Hammer curls

I know that muscle shape is genetic and cannot be changed, certain exercises do help emphesize (sp) certain parts of the muscle.
I read somewhere that doing hammer curls is not good because it will make the bottom part of the arm thicker and thus your peak will not be as prominent as it can be (assuming u have one). Since hammer curls works the brachiallus (sp). this does makes sence developing the brachailus will appear to reduce the space between the bicep and the elbow joint so less peak is formed when u flex?